spellck_extra_words attribute

This has the spellck lint recognize the attribute "spellck_extra_words" so that extra words can be added from the crate root. usage is like so:

```rust
//lib.rs
#![spellck_extra_words="these are not in word list but are now"]
```

problem: the attribute is marked unused by rustc. How can we have rustc recognize the attribute as used?

thank you for your time :)

closes #8 
This should be a normal attribute attached to the `extern crate`, not an inner attribute attached to the crate. Also, just looking now, could you put a blank line between the `spellck_extra_words` line and this one?